The diffraction process does not consist of a simple transmission or reflection of an incident beam, light or microwave, in a new direction different from that of the incident beam: the incident beam is split up into several beams, each redirected at a different angle according to different orders of diffraction. The percentage of incident light redirected in a given order of diffraction is the measure of the diffraction efficiency in that order. The efficiency of a diffractive component is determined by the surface profile of that element.
Efforts are typically made to obtain a maximum efficiency in a given order and to minimize the efficiency in the other orders. If the percentage of light which is not directed in the desired order of diffraction is substantial, then the result is parasitic light, transmitted or reflected depending on the type of component or a drop in the light flux in the preferred direction, which is detrimental to the quality of the component or of the system incorporating the component.
To optimize the efficiency in a single order of diffraction, it is known practice to produce a so-called “blazed” diffractive structure, that is to say one which has little or no light/wave diffracted in the orders other than the desired order, called blaze order.
Conventional diffractive elements are known, such as the echelette gratings illustrated in FIG. 1, exhibiting a period Λ which determines the angle by which the blaze order is diffracted, each period consisting of an element e called echelette. Also known are Fresnel lenses LF as illustrated in FIG. 2, having a central part Pc that is typically circular, and peripheral parts, rings A that are typically concentric (when the lens is in the axis).
The blaze effect sought is obtained by a gradual variation of the depth of a material of constant index n satisfying the blaze condition defined below.
The surface profile of these elements thus consists of continuous reliefs separated by discontinuities. These elements exhibit a maximum efficiency for a determined wavelength λ0, called blaze wavelength.
The blaze condition in the order 1 corresponds to a phase variation Δφ of a beam of wavelength λ0 incident on the component, between x=0 and x=Λ, equal to 2π.Δφ(λ0)=2π
For example, for an echelette grating by transmission, this condition is conventionally expressed(n−1).h0=λ0where n is the index of the material forming the grating and h0 is the height of the echelette.
FIG. 3 graphically illustrates the curve 15 of diffraction efficiency n(λ) in transmission of order 1 for a diffractive component in the blazed scalar domain in the order 1 as a function of the incident wavelength λ which is given by the formula:
                                          η            ⁡                          (              λ              )                                =                      sin            ⁢                                                  ⁢                                          c                2                            ⁡                              [                                  π                  ⁡                                      (                                          1                      -                                                                        λ                          0                                                λ                                                              )                                                  ]                                                    ⁢                                  ⁢        with        ⁢                                  ⁢                              sin            ⁢                                                  ⁢                          c              ⁡                              (                x                )                                              =                                    sin              ⁡                              (                x                )                                      x                                              (        1        )            
The light lost in the blaze order (order 1) is diffracted in higher orders. To take the example of a hybrid lens, which is composed of a refractive lens on one face and a diffractive lens on the other face and where the role of the diffractive face is to correct the chromatic aberrations of the refractive lens, this phenomenon results in the transmission of a parasitic light, which is detrimental to the quality of the image. That results in practice in the appearance of diffracted light in all the image plane. For example, the output image is not sharp, or is dull.
It is shown that this drop in efficiency originates from the weak dispersion of the material, which causes, for a small wavelength difference, the phase difference Δφ(λ) induced in the structure to diverge by 2π (whereas it is equal to 2π at the blaze wavelength λ0).
In the scalar approximation, that is to say typically for components having very large area widths on the wavelength scale, typically more than 100 times the wavelength, by disregarding the Fresnel losses, and by considering an incident beam at normal incidence, the phase difference as a function of the wavelength Δφ(λ) and the efficiency as a function of the wavelength η(λ) are in fact given by:
                              Δ          ⁢                                          ⁢                      φ            ⁡                          (              λ              )                                      =                  2          ⁢          π          ⁢                                          ⁢                                    λ              0                        λ                    ⁢                                    Δ              ⁢                                                          ⁢                              n                ⁡                                  (                  λ                  )                                                                    Δ              ⁢                                                          ⁢                              n                ⁡                                  (                                      λ                    0                                    )                                                                                        (        2        )                                          η          ⁡                      (            λ            )                          =                  sin          ⁢                                          ⁢                                    c              2                        ⁡                          [                              π                ⁡                                  (                                      1                    -                                          Δ                      ⁢                                                                                          ⁢                                                                        φ                          ⁡                                                      (                            λ                            )                                                                          /                        2                                            ⁢                      π                                                        )                                            ]                                                          (        3        )            where Δn(λ)=(n(λ)−nair) or (n(λ)−1), for a diffractive optical element etched in a material of refractive index n.
For these diffractive optical elements, it can be considered that Δn(λ)=Δn(λ0), because the dispersion of the material is negligible: the refractive index varies little around λ0. The equation (2) therefore becomes:
                              Δ          ⁢                                          ⁢                      φ            ⁡                          (              λ              )                                      =                  2          ⁢          π          ⁢                                          ⁢                                    λ              0                        λ                                              (        4        )            and the equation (1) seen above and represented in FIG. 3 is obtained, by replacing Δφ(λ) with this expression in (2).
Thus, the weak dispersion of the optical material in the conventional diffractive optical elements causes a drop in efficiency of the diffraction with the wavelength λ≠λ0 expressed by the equation (1). This variation of the phase difference with the wavelength is also present for the diffractive components operating by reflection.
These conventional diffractive optical elements are not therefore efficient in wide spectral band terms. A wide spectral band over which diffraction is desired can be characterized by a parameter Δλ/λ0, which is typically greater than 20% in the optical domain. For example, it is equal to 40% for thermal infrared, or equal to 100% for UV/VIS/NIR applications, and can even take the value of 130% in the photovoltaic domain. These conventional diffractive optical elements cannot be used in the optical systems dedicated to wide spectral band applications, such as gratings or lenses in optical instruments (transmission or reflection) or hybrid optical systems composed of refractive and diffractive optics.
Also known are other so-called binary microstructure diffractive optical elements, also called binary blazed gratings, or subwavelength diffractive optical elements (SWDOE), described in the publication “Broadband blazing with artificial dielectrics” OPTICS LETTERS, Vol. 29, No. 14, 2004 and the document WO 2005/038501. These binary blazed gratings are designed by producing a binary synthesis of the profile on a conventional diffractive optical element: the starting point is the conventional diffractive optical element that is to be synthesized and this grating is sampled to obtain points, with which an index or phase shift value can be associated. The sampling must be done with a period less than the design wavelength, to obtain a grating operating in subwavelength regime. The various computation techniques used are known to those skilled in the art and will not be recalled here. These techniques make it possible, for example, for an echellete blazed grating such as the grating RE represented in FIGS. 1 and 4a, to define a binary blazed grating as represented in FIG. 4b. To return to FIG. 4a, two echelons of an echelette grating RE of period Λ (or pitch of the grating) are represented. These echelons are etched in an optical material of index n.
A binary blazed grating corresponding to the grating RE of FIG. 4a is represented in FIG. 4b. The grating RE is sampled over a certain number of points at the period Λs chosen less than the blaze wavelength λ0. A certain number of points are obtained for each period Λ of the grating. Each point has correlated with it a given fill factor for a given microstructure type (hole, pillar): this fill factor is equal to the dimension d of the microstructure related to the sampling period of the grating: f=d/Λs. The fill factor of each microstructure is defined, by known computations, to locally give a phase-shift value Φ(x) similar to that of the echelette grating at the point sampled, and equal, as is known, to
            Φ      ⁡              (        x        )              =          2      ⁢              π        ⁡                  (                      n            -            1                    )                    ⁢              h        ⁡                  (          x          )                    ⁢              1        λ              ,where x is the coordinate of the point sampled on the axis 0x of the grating.
In the example of FIG. 4b, the binary microstructures are of pillar type. A set of binary microstructures is obtained which code the echelon pattern of the grating. This set of microstructures is repeated with the period Λ of the echelette grating of FIG. 4a. 
In the synthesis operation, a fill factor f is therefore defined for each microstructure which varies from one microstructure to the other to follow the phase function of the echelette grating. In the example, over each period Λ of the echelette grating (for each echelon), this dimension d increases with x. In practice, the fill factor f of a binary microstructure of the grating can take any real value lying between 0 and 1, including the values 0 and 1. For example, for the pillar p0 in FIG. 4b, the fill factor is 0.
FIGS. 5a and 5b show a conventional diffractive optical element of Fresnel lens type LF (FIG. 5a), and its binary synthesis by means of microstructures (FIG. 5b).
To describe the behaviour of a binary diffractive optical element, a concept of effective index neff is introduced to describe the interaction of the light on the microstructures. With this concept, the structure of the element is likened to a homogeneous artificial material, giving, on the component, an index profile comparable to a grating with effective index gradient, the effective index of which varies over the period Λ (or the portion) of the grating concerned. FIG. 4c schematically represents a grating with effective index gradient corresponding to the binary blazed grating of FIG. 4b. 
This concept of effective index and analytical formulae making it possible to calculate it are described in detail in various publications, of which the following can be cited: “On the effective medium theory of subwavelength periodic structures”, Journal of Modern Optics, 1996, vol. 43, No. 10, 2063-2085 by Ph. Lalanne, D. Lemercier-Lalanne, which shows in particular the curves of effective index variation with the fill factor and the incident wavelength (p. 2078).
In practice, the effective index is a function of the fill factor f (and therefore of the sampling period Λs) of the geometry of the microstructure, of the index n of the material (or, which is equivalent, of its permittivity ε) and of the incident wavelength λ. Different analytical formulae are thus known to those skilled in the art, which make it possible to calculate, for a given artificial material, the curves of variation of the effective index as a function of the fill factor f of the microstructures (therefore as a function of d and As) and as a function of the incident wavelength λ.
In practice, this concept is valid in all the cases where the sampling period Λs is substantially less than the structural cut-off value of the element, given by
            λ      0        n    ,where n is the refractive index of the material of the microstructures. This parameter gives the limit value of the sampling period, beyond which, for any fill factor, the material no longer behaves as a homogeneous material (thin layer), and for which work in subwavelength regime no longer applies. Beyond this value, there are many modes of propagation and many effective indices.
Substantially subwavelength propagation conditions are therefore assumed, with Λs≤1.5. λ0 (preferably
                    Λ        s            ≤                        λ          0                ⁢                                  ⁢        or        ⁢                                  ⁢                  Λ          s                    ≤                        λ          0                n              )    .Typically, in practice, Λs=λ0/2 or λ0/3 is generally chosen.
In these conditions, the blaze effect (that is to say the diffraction of the incident light in a single order of diffraction, the blaze order) is therefore obtained by variation of the optical index along the surface of the optical material. In effect, the microstructures are too small (subwavelength) to be resolved by the incident light (in terms of far diffraction field) which locally perceives an average index, the effective index neff. Over a period (grating) or a part (Fresnel lens), the use of sub-λ microstructures makes it possible to produce a phase law optimized for the energy radiated in the main deflected beam (blaze order) to be favoured, and the energy diffracted in the parasitic diffracted beams to be minimized.
The usual microstructures have geometries that are either hollowed out and of hole type, for example cylindrical, or protruding and of pillar type, for example with round, square, hexagonal or rectangular section. A combination of holes and pillars is also possible. The microstructures are of any form, preferentially with axes of symmetry to make them independent of the polarization of the incident beam at normal incidence.
Advantageously, they are arranged, within a period (grating) or a part (Fresnel lens), according to a sampling period Λs at least on the direction 0x of the surface plane of the grating or according to a radius starting from the centre for an in-axis lens. The structure of an off-axis lens is also applicable to the invention.
FIG. 6 illustrates an element 60 synthesizing a grating based on pillars P seen from above. In the example of 2D grating represented in FIG. 6 schematically, the mesh is square of dimensions Λsx=Λsy=Λs. In this example, there is a microstructure P for each mesh, for example at the centre of each mesh. The microstructures aligned in the direction 0X of the surface plane XY of the grating are assigned a fill factor varying progressively in a determined order, increasing or decreasing along the main direction 0X of the grating.
In the case of a synthesis of a grating of echelette (or multi-level) type, the microstructures aligned according to another dimension 0Y of the grating have an identical fill factor.
In the case (not represented here) of a synthesis of areas of a Fresnel lens, the fill factor of these microstructures can vary in all directions.
These binary blazed diffractive optical elements are known to exhibit efficiencies much greater than those of the conventional optics, and are used in the case of gratings with strong dispersion or for hybrid lenses with high numerical aperture.
The document WO 2005/038501 also describes a particular type of blazed transmission grating using the strong dispersion of the artificial materials to compensate for the variation of the diffraction efficiency as a function of the wavelength of the incident beam, in order to obtain blazed diffractive optical elements over a wide spectral band, that is to say diffractive optical elements that are efficient in their blaze order over a wide spectral band. These particular gratings, as illustrated in FIG. 7, use two different microstructure geometries, such as holes m1 and pillars m2. In a first portion, the microstructures according to the first geometry exhibit an effective index decreasing with the fill factor, and in a second portion the microstructures according to the second geometry synthesize an effective index increasing with the fill factor.
The composite artificial material 70 whose structure is illustrated in FIG. 7 exhibits an effective index gradient which varies between a minimum value neff/min and a maximum value neff/max. The minimum and maximum effective indices of the material are determined from curves of variation of the effective index with the fill factor of the microstructures.
A characteristic parameter of the material is the parameter α defined by:
                              α          =                                    (                                                δ                  ⁢                                                                          ⁢                                      n                                          m                      ⁢                                                                                          ⁢                      i                      ⁢                                                                                          ⁢                      n                                                                      -                                  δ                  ⁢                                                                          ⁢                                      n                                          m                      ⁢                                                                                          ⁢                      ax                                                                                  )                                      Δ              ⁢                                                          ⁢                                                n                  eff                                ⁡                                  (                                      λ                    0                                    )                                                                    ,                            (        5        )            where Δneff(λ0)=neff/max(λ0)−neff/min(λ0),
δnmin=neff/min(λ0)−neff/min(λ∞) and
δnmax=neff/max(λ0)−neff/max(λ∞).
With λ0 the design wavelength (or blaze wavelength) and λ∞ the great wavelength compared to the design wavelength λ0.
To obtain a grating which offers an optimal spectral width, it is necessary for the characterization parameter a to be strictly greater than 0, α>0 and preferentially 0.3≤α≤0.5.
The characteristics of the material 70 (index n, fill factor of the pillars and of the holes, sampling sub period, etc.) are therefore determined so as to obtain extreme effective index values culminating in the desired value of α.
The document WO 2005/038501 describes, by way of example, an effective index range between 1.5 and 2.1, with Λs=λ0/2. The sampling period Λs codes the area of period Λ equal to 25λ0, corresponding to 50 microstructures (35 holes and 15 pillars). The first point is coded by f=0 by a microstructure of hole type. The last point is coded by a microstructure of pillar type coded with f=0.68.
The element 70 has been produced in silicon nitride Si3N4 (n=2.1) with geometries of cylindrical holes with round section and of pillars with etched square section. The depth h of etching is 1.875λ0. The maximum effective index neff/max is coded using holes of zero diameter (neff/max=n=2.1) and the minimum effective index neff/min is coded using pillars of factor=0.68, i.e. of width d=0.34λ0. For this binary blazed grating with composite artificial material, α=0.39.
The curve of diffraction efficiency as a function of wavelength is given in FIG. 8: an area is obtained around λ0 that is fairly wide, in which the diffraction efficiency is, at its maximum, equal to 96% and remains above 90% between 0.6λ0 and 1.5λ0. The diffraction efficiency does not reach 100% in practice because of the discontinuities of the surface profile when switching from one type of geometry to another. At the discontinuity, there is a shadow effect and a phase discontinuity effect. The diffraction efficiency of this grating has been calculated for a grating period Λ equal to 25λ0. When there is a higher period, the effect of the discontinuities is lesser, and the efficiency is therefore better. With a period less than 25λ0, the effect of the discontinuities is greater and there is a loss in efficiency. A less good spectral width is obtained, but the improvement of the bandwidth may be satisfactory for certain applications. Thus, the use of composite artificial materials is not limited to components operating in the scalar domain. The concept of composite artificial material can be applied to the production of different subwavelength diffractive components, such as a grating illustrated in FIG. 9 or a Fresnel lens illustrated in FIG. 10.
However, the spectral width of the diffraction efficiency illustrated in FIG. 8 is not sufficient for certain applications such as spectroscopy and the production of optical or radiofrequency instruments for space (telescopes, etc.).
Also, the parameter a depends significantly on the material chosen, on the period and on the height of the structures, this having to take account of the fabrication constraints. In some applications, the choice of the material does not always lend itself to a parameter α>0.3, corresponding to a value lending itself to a use of the component over a very wide band. It is not therefore always easy to find all of the ideal conditions for the parameter α. It is often necessary to accept a slightly reduced α.
One aim of the present invention is to remedy the drawbacks mentioned by proposing a subwavelength diffractive component exhibiting a spectral width of its diffraction efficiency that is increased and a greater design flexibility to obtain a given efficiency curve mask η(λ).